A neutronic power unit was a piece of Dalek technology once employed in Antarctica. Approximately the size of an oil rig, it could produce enough energy to melt, according to Space Major Joel Shaw, "thousands of billions of tons of ice". Indeed, the Daleks once used it to flood most of the low-lying areas around the world.
Shaw, along with his fellow ADF agents Mark Seven and Reb Shavron, found the Dalek base and sunk it — after reversing its mode. The power unit thus presumably refroze that part of the polar ice cap that it had melted. (COMIC: Flood!!!)